The low-IF architecture is an attractive architecture for an integrated radio receiver as it enables a high level of integration of the channel filter. The frequency response of the channel filter for a low-IF receiver must be non-symmetrical about zero frequency by being capable of rejecting image frequencies and therefore a complex polyphase filter, having in-phase and quadrature-phase inputs and outputs, is required. See, for example, “A Low-IF, Polyphase Receiver for DECT”, B. J. Minnis et al, pp. I-60 to I-63, IEEE Int. Symposium on Circuits and Systems, May 28–31, 2000. Furthermore, differential inputs and outputs are desirable to provide protection from impulsive noise.
A basic building block for designing a filter is an integrator; see, for example “Top-down design of a switched-current video filter”, J. B. Hughes, pp. 73–81, IEE Proc. Circuits Devices Syst, Vol 147, No. 1, February 2000. It is well known that a bilinear form of integrator has performance advantages over other forms of integrator.
In order to reduce cost, it is desirable to implement a radio receiver or transceiver in a CMOS integrated circuit (IC). In such a receiver or transceiver, analogue and digital circuits are implemented in the same integrated circuit, rather than in separate integrated circuits produced by separate processes. As CMOS component dimensions are reduced to achieve higher levels of integration, the required supply voltage also reduces. Switched-current sampled analogue circuits are well suited to such a scenario as they offer a low power consumption and are able to perform well at low voltages.
Therefore there is a requirement for a complex switched-current bilinear integrator having differential inputs and outputs. A real switched-current bilinear integrator having differential inputs and outputs is disclosed in patent application EP 94306540.9, but not a complex version.
When designing circuits for processing complex signals a high level of matching between the in-phase (I) and quadrature-phase (Q) signal paths is normally required. Mismatch between in-phase and quadrature-phase signal paths in a polyphase filter will constrain the image rejection performance. Therefore there is a requirement for a complex switched-current bilinear integrator with protection from mismatch between signal paths.
One known technique that can be use to compensate for mismatch between signal paths is dynamic element matching (DEM) in which circuit elements are dynamically exchanged between signal paths so that different signal paths experience the same average circuit properties. See, for example “A Quadrature Data-Dependent DEM Algorithm to Improve Image Rejection of a Complex ΣΔ Modulator”, L. C. Breems et al, 2001 IEEE Int. Solid State Circuits Conf., paper 3.3. The use of dynamic element matching for a switched-current integrator has been suggested in patent U.S. Pat. No. 5,059,832 but in that patent no practical implementation was disclosed and no consideration was given to the problems that might arise in applying dynamic element matching to switched-current circuits.